The present invention relates to a process for determining the distillation characteristics of liquid petroleum products by express mini-distillation.
The volatility or more precisely the ranges of boiling temperatures of the different constitutive fractions of the petroleum products correspond to essential characteristic permitting characterisation of these products; these depend directly on the molecular weights of these fractions.
Currently the volatility of petroleum products is generally determined by physical tests under empirical conditions defined by standards, in particular the ASTM standards which are universally recognized by specialists.
By way of example, the standard ASTM D 86 permits determination of the volatility of light petroleum products having a boiling point lower than 400xc2x0 C., while the standards ASTM D 1160 and ASTM D 2892 also permit determination of the volatility of heavy petroleum products having a higher boiling point. In this last case, it is sometimes necessary to have recourse to distillation under vacuum to lower the boiling temperatures of the products analysed and thus avoid their decomposition.
Analysis apparatus suitable for these standards have been known for many years. These function schematically in accordance with the following principle:
The sample to be analysed is introduced into a distillation flask provided with an outlet tube and this flask is closed by a bung provided with a thermometer.
This outlet tube is connected to a condenser tube co-operating with a cooling system and the outlet orifice of which is situated over a graduated reception test-tube. The flask is heated under predetermined conditions in order to bring the sample to boiling and the vapours formed are collected in the reception test-tube after condensation.
The temperature of the vapours present in the distillation flask is noted for predetermined volumes of condensate collected in the reception test-tube.
The curve is plotted giving the percentage of the volume of sample collected as a function of temperature and from this curve, which characterises the sample, it is verified whether this is in accordance with the required specifications.
It is to be noted that the temperature indicated by apparatus functioning in accordance with these standards do not always correspond to the true boiling temperatures but may correspond to empirical temperatures, taking into account the test conditions, in particular the thermometers used.
These apparatus which are currently used by all specialists to characterise liquid petroleum products allow reliable results with good reproducibility to be obtained, which are therefore representative of the sample analysed, but they do however have many disadvantages: they are in fact particularly heavy and bulky; moreover, the volume of sample required to perform a volatility test is relatively large (of the order of 100 ml) and the duration of each test is not less than 45 minutes.
To correct these disadvantages, in accordance with Byelorussian publication 198 0 801, researchers at the State University of Polotsk have already proposed a process and an apparatus permitting determination of the distillation characteristics of liquid petroleum products by physical tests each only lasting approximately 10 mn and each only requiring a much reduced volume of sample (of the order of 5 to 15 ml).
Another advantage of this process and this apparatus is linked to the fact that the characteristics of the product analysed are determined directly from temperature and pressure measurements and therefore they require no measurement of the volume of condensate collected in a reception test-tube.
The apparatus in accordance with this prior publication includes schematically a distillation flask co-operating with heating organs and which is provided with a capillary tube at the level of its outlet tube and is closed by a bung provided with a temperature sensor which is immersed in the boiling liquid and a differential sensor allowing measurement of the pressure in the vicinity of the capillary tube inlet.
The process employed upon use of this apparatus is based on a particular algorithm which permits calculation of the temperature of the vapour of the sample from its temperature of the vapour of the sample from its temperature in the liquid state in the distillation flask and from the rise in pressure within the latter consequent upon the presence of the capillary tube.
This method of determination of the distillation characteristics of the petroleum products has certain advantages.
However, the curves thus obtained have insufficient reliability taking into account the fact that the barometric pressure variations together with the residues in the distillation flask and the losses of sample in the vapour phase are not taken into consideration.
Moreover, the heating organs interfere with the measurement values of the temperature of the sample in the liquid phase.
Moreover, this method is only suited to the determination of the distillation characteristics of light liquid petroleum products having boiling temperatures lower than 400xc2x0 C., and cannot be used for the determination of the distillation characteristics of heavy liquid petroleum products, since, operating at atmospheric pressure, they would cause thermal decomposition of these products.
The essential disadvantage of this method is however linked to the fact that the tests performed by it are not in correlation with the standards and in particular with the ASTM standards, which constitutes a major fault as these standards are currently universally recognised by specialists in the field of analysis of petroleum products.
The present invention has the object of correcting these disadvantages by proposing a process permitting determination of the distillation characteristics both of light liquid petroleum products and of heavy liquid petroleum products and their mixtures by express mini distillation so as to provide reliable and reproducible results in correlation with those from tests in accordance with the standards and in particular with the ASTM standards universally recognised by specialists.
In accordance with the invention, it has been possible to attain this end by means of a process including the following steps:
1. a volume of the order of 5 to 15 ml of the sample to be analysed is introduced into a distillation flask co-operating with a heavy element at its lower part and provided with a pressure detector and two inertialess temperature sensors permitting measurement on the one hand of the true value of the temperature of the sample in the liquid phase and on the other the true value of the temperature of the sample in the vapour phase at a level situated a little below the inlet of the outlet tube with which the distillation flask is equipped,
2. the distillation flask is heated with a constant heating intensity depending on the nature of the sample to be analysed so as to progressively boil it,
3. the vapour pressure in the distillation flask is constantly measured at the level of the inlet of the outlet tube, as are the true values of the temperatures of the sample in the liquid phase TL and the vapour phase TS and the curves are plotted representing the variations in the pressure and these temperatures as a function of time xcfx841,
4. the primary and secondary derivatives are established of the curve representing the variations of the temperature of the sample in the liquid phase and in the vapour phase:             ⅆ              T        S                    ⅆ              τ        1              ;      xe2x80x83    ⁢            ⅆ              T        L                    ⅆ              τ        1              ;      xe2x80x83    ⁢                    ⅆ        2            ⁢              T        S                    ⅆ              τ        1        2              ;      xe2x80x83    ⁢                    ⅆ        2            ⁢              T        S                    ⅆ              τ        1        2            
and the temperature is deduced of initial boiling in the liquid phase TLIBP which corresponds to the point for which                               ⅆ          2                ⁢                  T          L                            ⅆ                  τ          1          2                      =    0    ,
5. the temperature of initial boiling in the vapour phase TSIBP is determined which corresponds to the time for which a commencement of increase of pressure is observed,
6. the temperature TLEND is determined for which the value indicated by the temperature sensor which measures the true value of the temperature of the sample in the liquid phase corresponds to the value indicated by the sensor which measures the temperature of the sample in the vapour phase and by convention this temperature TLEND is considered to be equal to the final boiling temperature in the vapour phase TSFBP,
7. the volumetric percentage of sample distilled xcexdv is determined from curves representing the variations as a function of xcfx841 in the pressure of vapour P in the distillation flask and in the true value of the temperature of the sample in the vapour phase TS by the function:       v    Vi    =      [                            Sf          ⁢                      (                                          Ti                S                            ,              Pi                        )                                    Sf          ⁢                      (                                          T                S                            ,              P                        )                              ,              V        i        res              ]  
in which Sf(TS, P) depends on the area situated below the pressure curve in the distillation process while Sf(TiS, Pi) depends on the fraction of this area at the time xcfx84li, and Vires represents the volume of liquid in the distillation flask at the time xcfx84li,
8. the molar percentage of sample distilled VM as a function of the true value of the temperature of this sample in the vapour phase TS is determined by the function Vml=f(xcexdv1, xcfx81i, TSi) in which xcfx81i represents the molar density at the time xcfx84li,
9. the final boiling temperature in the liquid phase TLFBP is determined by iteration in accordance with the formula:       T    FBP    L    =            T      END      L        +                            (                                    T              END              L                        -                          T              FBP              L                                )                          [                                                    v                END                                            (                                  1                  -                                      v                    END                                                  )                                      /            a                    ]                    ⁢              xe2x80x83            ⁢              1        /        k            
in which xcexdEND represents the molar percentage of sample distilled at the temperature TLEND and a and k are coefficients in the mathematical model of distillation corresponding to the empirical formula:       v    M    =            a      ⁢              xe2x80x83            ⁢              τ        k                    (              1        +                  a          ⁢                      xe2x80x83                    ⁢                      τ            k                              )      
calculated by iteration from the equation:       ln    ⁢          xe2x80x83        ⁢          (                        v          Mi                          1          -                      v            Mi                              )        =            ln      ⁢              xe2x80x83            ⁢      a        +          k      ⁢              xe2x80x83            ⁢      ln      ⁢              xe2x80x83            ⁢              τ        i            
in which:       τ    i    =                    T        i        L            -              T        IBP        L                            T        FBP        L            -              T        i        L            
calculating at each step a new value of TFBPL up to the moment when:
TFBPL(n)xe2x88x92TFBPL(nxe2x88x921)xe2x89xa61xc2x0 C. 
10. the molar percentage of sample distilled as a function of the true value of the temperature of this sample in the vapour phase is recalculated so as to take account of the residues and losses of sample in the vapour phase by the formula:
xcexdmi=xcexdxe2x80x2Mi+xcex94Li+xcex94Si 
in which xcex94Si represents the percentage of vapour phase in the course of distillation, xcex94Li the percentage of liquid phase at the moment of its formation by condensation and xcexdMi the molar percentage of sample distilled taking the residues into consideration,
11. the volumetric percentage of sample distilled xcexdVi as a function of the true value of the temperature of this sample in the liquid phase is determined by the formula:
xcexdVi=F(xcexdMi, xcfx81i, TiL) 
12. and the corresponding curve is plotted.
It is to be noted that the mathematical model of distillation corresponding to the empirical formula:       v    M    =            a      ⁢              xe2x80x83            ⁢              τ        k                    (              1        +                  a          ⁢                      xe2x80x83                    ⁢                      τ            k                              )      
is detailed in the publication Dimudu T. A., Jarkova O. N. and Abaev G. N. Mathematical model of fractional distillation of petroleum products and its identification by experimental data // Inzynieria Chemiczna i Procesowa, 1996, V.17, N 4.
It is moreover to be noted that the experimental data obtained by the above-mentioned process correspond to a standard distillation technique in a column with one single plate called LBD distillation (Laboratory Batch Distillation) or FD distillation (Fractional Distillation).
In accordance with another characteristic of the invention, the heating intensity of the heating element is adjusted depending on the nature of the sample to be analysed so that the time necessary for distillation of it is of the order of from 5 to 15 minutes.
In the case of a sample having totally unknown characteristics, this power can where necessary be determined in a preliminary distillation step.
In accordance with another characteristic of the invention, the empirical value Tstand of the temperature of the sample in the vapour phase corresponding to a standard is determined from the value TL of the temperature of the sample in the liquid phase calculated at steps 1 to 12 by the formula:
TiSTAND=TiLxe2x88x92xcex8i
in which xcex8 is a function which represents the difference between these temperatures.
The process in accordance with the invention is particularly well suited to determining the distillation characteristics of light liquid petroleum products in correlation with the standard ASTM D 86.
In this case, and in accordance with another characteristic of the invention:
TSTANDD 86is the empirical temperature corresponding to the standard ASTM D 86 and xcex8iD86 is calculated from the function:       θ    i    D86    =      f    ⁡          (                                    ⅆ                          T              i              L                                            ⅆ                          v              Vi                                      ,                  T          i          L                ,                  v          vi                    )      
and is determined either graphically, or from values of the parameters (a, k, TIBPL, TFBPL), calculated in steps 1 to 12.
The process in accordance with the invention can also be adapted to determining the distillation characteristics of heavy liquid petroleum distillation products having boiling temperatures higher than 400xc2x0 C., at atmospheric pressure, without reaching temperatures for which there would be a risk of coming up against thermal decomposition of the analysed product.
To this end, and in accordance with another characteristic of the invention:
a light vehicle liquid petroleum product is selected having a boiling temperature below 300xc2x0 C. compatible with the sample to be analysed,
this vehicle liquid is subjected to steps 1 to 12 so as to obtain the curve representing the molar percentage VM of vehicle liquid distilled as a function of the true temperature of this liquid in the liquid phase TL xcexdMi(vehicle)=f (TiL),
a mixture is prepared containing approximately 85 to 95% of vehicle liquid and 5 to 15% sample to be analysed so that at least 90% of this mixture has a boiling temperature lower than 360xc2x0 C.,
this mixture is subjected to steps 1 to 12 so as to plot the curve representing the molar percentage VM of this mixture distilled as a function of the temperature TL of the mixture in the liquid phase xcexdMi(mix)=f(TiL) in the same system of co-ordinates as the curve xcexdMi(vehicle)=f(TiL),
it is considered by convention that the final boiling temperature TFBPHP of the sample to be analysed is equal to the final boiling temperature of the mixture in the liquid phase,
TFBPHP=TLFBP (mix) 
TLFBP (mix is determined by iteration in accordance with steps 1 to 9,
the initial boiling temperature in the liquid phase TIBPHP of the sample to be analysed is calculated by the formula:       T    IBP    HP    =            T      1        -          Δ      ⁢              xe2x80x83            ⁢                                    T            1                    ⁡                      [                                          v                M1                                                              a                  HP                                ⁡                                  (                                      1                    -                                          v                      M1                                                        )                                                      ]                                    1          /                      k            HP                              
in which T1 represent the temperature of intersection of the curves xcexdMi(vehicle)=f(TiL) and xcexdMi(mix)=f(T1L), xcexdM1 the molar percentage of sample which corresponds to this temperature T1 et xcex94T1=TFBPHPxe2x88x92T1
aHP and kHP are determined using the system of additive equations:
xcexa3 xcexdMiSi (TL, ai, ki)=Smix (TL, amix, kmix) and 
xcexa3 xcexdMiSi (xcfx84)=Smix (xcfx84) 
in which Si(T) and Si (xcfx84) are function of the respective areas situated below the distillation curves in the systems of co-ordinates xcexdM, T and xcexdM, xcfx84 and xcexa3 is a function of the specific gravity of the vehicle product in the mixture, and the curves xcexdMHP=f(T) are plotted from the formula:       v    M    HP    =                              a          HP                ⁢                  τ                      k            HP                                      (                  1          +                                    a              HP                        ·                          τ                              k                HP                                                          .  
For the sake of simplification, a well known product such as kerosene and/or a liquid petroleum product having a boiling point lower than 300xc2x0 C. is generally used a vehicle.
In addition to the normal LBP or FD distillation techniques, the specialists in the field of analysis of petroleum products sometimes have recourse to distillation techniques corresponding to columns with a plurality of theoretical plates, generally at least fifteen theoretical plates called TBP (True Boiling Point) true distillations, which are also defined by universally recognised standards.
Now, the present invention also permits plotting of the curves of the TBP true boiling points of a sample, in particular in order to know its composition.
To this end, and in accordance with another characteristic of the invention:
from LBP standard distillation curves xcexdM=f(T) corresponding to a column with a single plate, the curves of the TBP true boiling points are plotted corresponding to a technique with a column with at least 15 theoretical plates, considering by convention that the temperatures TLEND (LBP) and TLEND are equal, that in the system of co-ordinates xcexdM, T the surfaces situated below the LBP standard distillation curves are equal to the areas situated below the curves of the TBP true boiling points and that TIBP(TBP)=f(SLBP), f(SLBP) depending on the area situated below the LBP standard distillation curve in the system of co-ordinates xcexdM, T.
The invention also relates to an apparatus permitting implementation of the above-mentioned process.
In accordance with the invention, this apparatus is characterised by the fact that it includes
a distillation flask dimensioned to receive from 5 to 15 ml of a sample to be analysed and provided at its upper part with a bung and a lateral outlet tube co-operating with a condenser,
organs for heating the distillation flask, at its lower part, with an adjustable constant heating intensity,
two inertialess temperature sensors introduced into the distillation flask through tubes passing through the bung so as to permit continuous measurement, on the one hand of the true value of the temperature of a sample in the course of distillation in the liquid phase and on the other the true value of the temperature of this sample in the vapour phase at a level situated a little below the inlet of the outlet tube,
a device for continuous measurement of the pressure in the vapour phase of a sample in the course of distillation which includes a pressure detector connected to the internal part of the distillation flask by a pipe passing through the bung and a capillary tube introduced into the internal part of the outlet tube at the level of the inlet of this tube, and
organs for reception and exploitation of signals transmitted by the temperature sensors and the pressure detector.
Taking these characteristics into account, to perform a test the user introduces 5 to 15 ml of sample to be analysed into the distillation flask, for example by means of a syringe, and the recloses this before selecting a heating intensity.
The organs for reception and exploitation then automatically switch the circuits associated with the temperature and pressure differential sensors and the heating and distillation of the sample commence.
During distillation, the different sensors continuously transmit to the organs for reception and exploitation signals which enable them to automatically create the distillation curves and display these curves on a screen and print them, this in a time less than fifteen minutes.
Depending on the field of the user for whom it is intended, the distillation flask, without thereby departing from the scope of the invention, may be either a permanent flask made in particular of glass or stainless steel, or a disposable flask, in which case the capillary tube is formed by a fixed element preferably made of stainless steel.
In accordance with another characteristic of the invention, the apparatus forms a portable, one-piece assembly.
This is a particularly advantageous characteristic as it allows an apparatus to be obtained of much reduced weight and volume able to very rapidly perform tests on petroleum products on any site, in particular in theatres of operation in the military field.
The characteristics of the process and the apparatus which form the object of the invention will be described in more detail with reference to the attached FIG. 1, which is a diagrammatic view showing an example of configuration of this apparatus.
As shown in FIG. 1, this includes a distillation flask 1 of spherical form provided with a lateral outlet tube 4 which is dimensioned to receive 5 to 15 ml of a sample to be analyzed.
This flask 1 is heated at its lower part with an adjustable constant heating intensity by a heating resistance 2 and is hermetically closed at its upper part by a removable bung 3 through which the sample to be analysed can be introduced, in particular by means of syringe.
Two inertialess temperature sensors 5, 6 are introduced into the distillation flask 1 through pies 5xe2x80x2, 6xe2x80x2 passing through the bung 3.
The first temperature sensor 5 is immersed in the liquid to be analysed to allow continuous measurement of the true value of the temperature of this sample, in the course of distillation in the liquid phase.
The second distillation sensor 6, for its part, is mounted at the upper part of the distillation flask 1, at a level situated a little below the inlet 40 of the outlet tube 4, so as to allow continuous measurement of the true value of the temperature of the sample in the course of distillation in the vapour phase.
The apparatus also includes a device for continuous measurement of the pressure in the upper part of the distillation flask 1.
As shown in FIG. 1, this device essentially consists of a pressure differential sensor 7 which is connected to the internal part of the distillation flask 1 by a flexible pipe 7xe2x80x2, also passing through the bung 3.
The differential pressure sensor 7 co-operates with a metal capillary tube 8 introduced into the internal part of the outlet tube 4 of the distillation flask 1, so that the vapours of sample in the course of distillation escape to the outside through this capillary tube 8.
This is mounted so as to open into the distillation flask 1 at the level of the inlet 40 of the outlet tube 4 and is connected at its opposite end to an air condenser 9 permitting condensation of the vapours escaping from the distillation flask 1 to transfer them into a recovery element which is not shown.
The configuration of the outlet tube 4 of the distillation flask 1, of the capillary tube 8 and of the condenser 9 are so selected that the condenser which is screwed in during assembly of the apparatus compresses the capillary tube and creates the seal at the level of the exit of the vapours from the distillation flask 1.
Moreover, the pipe 7xe2x80x2 for connection of the pressure differential sensor 7 and the internal part of the distillation flask 1 is provided with a T joint 10 including a flow limiter which is not shown at its internal part; to this joint 10 is connected an auxiliary pipe 11 connected to a micro-compressor 12 so as to blow a small flow of air into the pipe 7xe2x80x2, allowing prevention of the signals emitted by the pressure differential sensor 7 from being falsified by the introduction of condensate into the pipe 7xe2x80x2.
A fan 13 allows cooling of the distillation flask 1 after each test.
As shown diagrammatically in broken lines in FIG. 1, the signals emitted by the differential pressure sensor 7 and by the first and second temperature sensors 5 and 6 are transmitted to organs for reception and exploitation 14 of these signals which print and display the distillation curves on a screen in response.